1. Field of the Invention
The present invention relates to a wrist mechanism having two, three or more degrees of freedom and used by being disposed at the tip of a robot arm in an industrial robot.
2. Description of the Prior Art
As is well known, the major role of a wrist mechanism disposed at the tip of a robot arm is to change the posture of an object to be controlled, such as a hand, a spray gun, and a welding torch, which is secured to the wrist mechanism, and the mechanism has the functions of bending, oscillating, and twisting. The power for driving such a wrist mechanism is, in many cases, transmitted by making use of a gear train. In the case of utilizing a gear train, however, it is indispensible to adjust the backlashes of gears, and noise caused by the engagement of gears also becomes a problem.
An arrangement in which the necessary power is transmitted by making use of universal joints instead of a gear train is disclosed in Japanese Patent Unexamined Publication No. 60062/1978, according to which adjustment of backlashes becomes unnecessary and the problem of noise can be alleviated. However, the angle of intersection of input and output shafts in each universal joint changes according to a bending movement, and the universal joint is also bent at a sharp angle as the amount of the bending movement increases. This causes a burden to the universal joint, resulting in undesirably restricting the amount of its bending movement.